The invention relates generally to valves and more particularly to valves having back seat arrangements as a secondary sealing means.
It is well known to provide a valve operator with a back seat capability whereby under predetermined conditions a fluid-tight seal may be established between cooperative seating surfaces on the interior of the valve casing and the valve stem. The back seat capability is desirable as a secondary sealing means which can be placed in operation in the event of a loss of sealing normally provided by the valve stem packing. This capability is also useful when it is desired to replace or repair a valve stem packing. For example, U.S. Pat. No. 855,385 teaches the concept of back seating of a rising stem plug valve wherein raising of the valve element to the full open position by means of a handwheel accomplishes a metal-to-metal seal between the interior of the valve casing and the valve spindle. The back seating is intended for use when it is desired to replace the valve stem packing gland without removing the valve from service and shutting off the flowline.
In U.S. Pat. No. 4,245,661 a valve is disclosed wherein the normally non-rising stem operator and its bearing enclosing bonnet gland can be manually raised so that the stem packing can be removed and replaced without disturbing the valve installation. The patent also discloses the use of a eutectic material which is subject to melting at a predetermined high temperature whereby the valve stem will automatically rise and establish a back seat to preclude a possibility of leakage due to a stem packing failure. As is true with most examples of valves with back seating capability provided by use of a eutectic material, the automatic back seating of the valve stem requires the condition of an extremely high temperature such as occurs with a fire. It is also a characteristic of manually operated valves that back seating becomes operative only when the valve is in either the fully closed or fully open condition, but not both. Further, since the back seat is connected to the stem in the prior art devices, whether the valves are operated by handwheels or actuators, it does not become operative should the valve stem become locked in place as in the event of a malfunction to a pneumatic or hydraulic valve actuator or a handwheel drive mechanism. However, in no instance does back seating occur directly as the result of loss of integrity of the valve stem packing.
It is therefore a purpose of this invention to provide a back seat capability for valves which is independent of valve stem movement or valve element position for accomplishing a fluid-tight seal between a valve casing and the valve stem in the event of leakage around the stem packing and which is adapted for use with either rising stem or non-rising stem valves.
It is another object to provide a pressure energizable back seat means for valves which is unconnected with the valve stem and is operative independently of the valve stem position for quickly accomplishing a seal between the valve casing and the valve stem in the event the valve is exposed to an extremely high temperature condition and there is a failure of the stem packing.